Information classification is a crucial aid
in the interpretation of the relationships between categories
(1). The classification of vegetation into units facilitates
communication between researchers and resource managers
(2,3). For wetland managers and researchers the Cowardin et al.
(4) classification system has become an
integral component of wetland inventory and conservation efforts
(5). The goals set forth by Cowardin et al.
(4) include (a) the description of ecological units with
homogenous natural attributes, (b) arrangement of those attributes in order to aid
resource managers, (c) identification of units
for classification and inventory, and (d) provision of uniformity in concepts and
terminology (5). At the base of the
Cowardin et al. (4) classification are the
dominance types which provide a description of vegetation units within a region.

Information on wetland
dominance types for a region or state can be
obtained through field studies or through a review of the published literature.
Unfortunately, there is a paucity of literature regarding
the vegetation of western Oklahoma herbaceous wetlands. The need for
increased quantitative data for wetland vegetation
in Oklahoma has been noted (6). Recent studies of western Oklahoma wetlands
do not provide a classification of dominance types (7,8).
Penfound (9) recognized 27 wetland vegetation types in western
Oklahoma in a study of vegetation associated with reservoirs and natural lakes.
The results of this study were based presence/absence data but were not
quantitatively analyzed.

The structure and dynamics of
vegetation in buffalo wallows is probably the most extensively studied wetland habitat
in western Oklahoma (10-14). Buffalo
wallows are small-scale landscape features with
low species diversity and with vegetation composed of primarily perennial
wetland species (14). Vegetation structure of
playa lake wetlands were analyzed by Hoagland and Collins
(15), including sites in Texas and Cimarron Counties.

The goals of the current study were
to provide a quantitatively derived classification and description of
herbaceous wetland vegetation. Such a
classification will be of value to wetland conservation
and management efforts in Oklahoma.

Study area: Study sites were located in
16 western Oklahoma counties (Fig. 1). Within the study area, latitudinal variation
in temperature and longitudinal variation in precipitation combine to produce a
south-

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Figure 1: Location of western Oklahoma emergent wetland study sites. The number
of sites quantitatively sampled in each is numerically designated.

east-to-northwest environmental
gradient (16). For example, mean annual
temperature in the southeastern portion of the study area is
17.2oC and in the northwest is
13.3oC (17). Likewise, the mean annual
low temperature decreases from 18oC in
the southeast to 1.7oC in Cimarron County,
and the mean annual high temperature from
28.9oC to 25.6oC. There are 200 frost
free days in Jefferson County as contrasted with 170 days in the northwest
(17). Mean annual precipitation ranges from 83 cm to 41
cm along a southeast to northwest axis.

Physiographically, the study is
characterized by gently rolling topography with local occurrences of dissected hills.
The surface geology is composed primarily of shallow-marine deposits
(18). However, Tertiary sands, clays, and gravels
are prominent in Ellis, Harper, and the Panhandle Counties
(18). Several soil great groups are present in the study area,
ranging from darkly colored loams and clay loams developed under mid and
shortgrass prairies which are typical of the
panhandle, to dark or dark-reddish clay and
clay-loam developed under tall, mid, and
shortgrass prairie which are prominent in the
remainder of the study area (19). Stabilized
and active sand dunes along major streams are another important group of soils. They
are mostly brown and light brown loams and sands with clay lenses that support
semi-permanently flooded habitats (19).
Gypsum outcrops and saline springs produce soils that support halopyhtic vegetation
(20-22). In excess of 40 hydric soil types have
been identified in western Oklahoma (8).

Data collection and
analysis: Study sites were located by review of 1:24,000
scale US Geological Survey topographic quadrangles and National Wetland
Inventory maps. Sites were excluded from this
study if they met any of the following criteria:
(a) heavily grazed (i.e., vegetation cropped to less than 3 cm and trampled), (b)
converted to exotic pasture grasses (i.e.,
Cynodon dactylon), (c) currently or recently in
row crop production, or (d) evidence of extensive physical modification. In
the field, sites were assigned to the appropriate class in the Cowardin et al.
(4) system. Quantitative vegetation data were
collected from acceptable sites using randomly placed 0.25
m2 quadrats. This sample size was chosen because of the small size
and linear nature of many western Oklahoma wetlands. A larger sample size
would increase the amount of upland vegetation in the quadrat. The number of
quadrats sampled depended on the total area of the site. All species occurring within a
quadrat were recorded, and percent cover was then visually estimated to the nearest 5%.

Vegetation data from each site
were compiled into a matrix of mean species cover values-by-site. Vegetation data
were classified into floristic associations by
using two-way species indicator analysis
(1,23). These data were subsequently analyzed by

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using detrended correspondence
analysis (DCA; 24,25) to identify regional
gradients and trends in species composition.

Vegetation classification: A total of 55
sites were sampled and 182 species encountered. The greatest number of sites where
located in Blaine and Cimarron Counties. The distribution of sites reflects the
availability and access to sites which met the
sampling criteria. Table 1 lists all species
encountered during the sampling, the TWINSPAN clusters into which they were classified,
and species richness, diversity, evenness values for each cluster. TWINSPAN
analysis produced eight vegetation clusters (Table 2).
Echinochloa crusgalli, Eleocharis palustris, Polygonum pensylvanicum, Rumex
altissimus, Schenoplectis americana, S.
tabernaemontanii, and Typha domingensis were the
most frequently encountered species. All vegetation clusters were dominated by
perennial plant species.

Cluster 1 was composed of pond
sites that occurred in the swales of stabilized
sand dunes. These ponds and attendant vegetation were encountered throughout
central Oklahoma, but not in far northwest Oklahoma or in the Panhandle. The
dominance type at these sites was described as
Heteranthea limosa-Bacopa
rotundifolia-Marsilea vestita. Associated species included
Ammania coccinea, P. pensylvanicum,
P. coccinea, and Sagittaria
latifolia. The most abundant plants on pond shorelines were
Leptochloa fasicularis and E.
crusgalli. Upland vegetation was dominated by
Artemisia filifolia, Sporobolus
cryptandrus, and Schizachyrium
scoparium. Several swales throughout the study area contained this vegetation
type, but were not sampled because of plowing.

Vegetation of Clusters 2 and 6
occupied small depressions on clay soils, and the dominant species in both were of the
genus Eleocharis; E. compressa in the case of
Cluster 2, and E. palustris in Cluster 6. Cluster 6
had both the lowest species richness (17) and species diversity value (1.07) of the
TWIN-SPAN clusters. In both cases, upland vegetation was of the mixedgrass
prairie dominated by S. scoparium-Sorghastrum nutans.

Cluster 3 contained the greatest
number of sites and the highest species diversity.
The greatest mean cover value and highest frequency was
T. domingensis. Frequently co-occurring species included
Lippia nodiflora, P. lapathifolium,
P. pensylvanicum, Panicum virgatum,
Salix nigra, and S. americanus. Sites for this vegetation occurred
in a broad array of habitats, including palustrine and riverine.

Cluster 4 was dominated by
S. americanus. Frequently occurring species
included Cephalanthus occidentalis, Lycopus
americana, P. virgatum, and S.
tabernaemontanii. No clear vegetation type emerged from
cluster 5. Species with high cover values in this cluster included one site dominated
by Nelumbo lutea and two by Thalia
dealbata.

Cluster 7 contained playa lakes.
Pascopyrum smithii, a mesic C3 grass, was
the dominant species at these sites. L.
cuneifolia and Oenothera canescens co-occurred in
all playa lake. Guymon playa, which captured irrigation run-off, was the only
playa dominated by obligate wetland species (P.
coccinea and P. lapathifolium).

Sites in Cluster 8 occurred in
saline areas and were dominated by Distichlis stricta-S.
americana. Frequently occurring species included
Aster subulatus, E. palustris, and
L. lanceolata. Boggy Creek salt flats, located in a gypsum outcrop area in
Beckham County, a unique salt tolerant species,
Cressa truxillensis, which had been previously documented from only one site
in Oklahoma and not collected since the 1950
(26).

Ordination: Species turnover
was lower on the first DCA axis (s.d. = 3.54) than the second DCA axis (s.d. = 6.71) (Fig. 2).
Eigenvalues were high for both axes, 0.91 and 0.71, respectively. One site, the only
site containing Phragmites australis, exerted
a strong outlier influence and was excluded from analysis. The first axis represents
a gradient of decreasing hydroperiod. Floristically, sites were separated by
dominance of Typha spp. and/or
Schoenoplectus spp. or grasses.
Typha spp. and Schoenoplectus wetlands occur in semi-permanently
flooded to permanently flooded conditions, while playa lakes, by comparison, were
in-

Five of the seven vegetation
associations correspond with the 29 vegetation
associations listed by Penfound (9) for
western Oklahoma; E. palustris, T. domingensis,
S. americanus, N. lutea, and D. stricta.
D. stricta vegetation is a dominant type at the
Great Salt Plains (22). Several of the
vegetation types listed by Penfound (9) represent
stages in the annual hydrologic dynamics of wetlands. For example, six wetland
types were listed for playa lakes:
Lythrum-Verbena, Buchloë
dactyloides, Chenopodium-Myosurus, P.
persicaria, and Sida-Triticum. All of
these species and genera, with the exception of
Myosurus, were encountered to varying degrees of abundance, although none
were dominants. B. dactyloides, which is
capable of withstanding spring flooding
(27), occurred in several of the playa
lakes sampled for this study, but was not a dominant. It was reported as a
dominant in playa lakes located in adjacent Texas
and New Mexico (6).

Wetland vegetation dominated
by members of the Cyperaceae, particularly the genus
Eleocharis, have been reported from other studies. In this study, three of the
vegetation types reported were dominated by the Cyperaceae. The genus
Eleocharis is an important component of buffalo
wallow vegetation (10 - 14,28). In western
Oklahoma, Eleocharis species were most
abundant in wet depressions or along shorelines. Only Penfound
(9) reported vegetation dominated by S.
americanus, which is interesting when its broad distribution
in western Oklahoma is considered.

There were several sites dominated
by broadleaf wetland plants. Nelumbo lutea,
the most broadly distributed member of Nymph-aceae in Oklahoma, dominated
one site. It has been reported as a dominant in other parts of Oklahoma
(29,30). Interestingly, T. dealbata was the
dominant at a Jefferson County site. The Atlas of
the Flora of Oklahoma database shows the nearest population of
T. dealbata to be located in Johnston County, making the
Jefferson County station the westernmost in Oklahoma. Hoagland
(6) reported the co-dominant species to be
T. latifolia; however, in this study, Justicia
americana was co-dominant.

This study demonstrates the
diversity of vegetation in western Oklahoma emergent wetlands. However, further study
is needed. The inclusion of aquatic vegetation as well as woody riparian vegetation
is warranted. Although in many areas woody riparian wetland vegetation has
been displaced by the Tamarix spp., stands of
Salixnigra and S.
exigua are extensive. Such efforts would complement this work
by completing a classification of wetland vegetation.

ACKNOWLEDGMENTS

Partial funding for this research
was supplied by a grant from the Fish and Wildlife Foundation. The field
assistance rendered by Newell A. McCarty, Debby Benesh, Sudeep Chandra, Carter Miller, and Stephanie Love is greatly appreciated.